In these digital cellular telephone systems, three main functions must be carried out for processing the base band.
1--Speech coding/decoding;
2--Channel coding/decoding (CRC/parity coding, convolution coding, interlacing, deinterlacing, Viterbi decoding, parity check/CRC);
3--Modulator/demodulator, modem.
Currently, these functions are implemented in three separate functional units.
By reason of the time division multiple access method (TDMA), the operating time ratio of some of these units is very low.
For example, the modulator which is an element whose task is indivisible is implemented with a dedicated operator activated for a constant period of time.
In the GSM system, the maximum value of this ratio for the modulator is 12.5%, but overall it is less than 0.2%.
In current implementations, the circuitry resources necessary for this operator cannot be re-used to carry out other tasks during the periods of inactivity.
This digital modulator operator is usually implemented within linear parts, close to converters and requires a mixed digital/linear technology which is a penalty on the digital part.